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Paying customers will still get their money's worth, though, through access to extra features: rare items, more storage space in your backpack (300 slots compared to 50 for free accounts), the ability to give and receive gifts, and improved special abilities such as crafting and trading.
All other aspects of the game, however, are the same. To convert your free account to a premium account, simply make a purchase of any amount in the in-game Mann Co. store.
Check out a short video announcement for the new, free Team Fortress 2, which will surely get you hyped up for the game.